Reference Site: Google Support https://support.google.com/a/answer/174124?hl=en
Email spoofing is when email content is changed to make the message appear from someone or somewhere other than the actual source. Spoofing is a common unauthorized use of email, so some email servers require DKIM to prevent email spoofing.
DKIM adds an encrypted signature to the header of all outgoing messages. Email servers that get signed messages use DKIM to decrypt the message header and verify the message was not changed after it was sent.